Polycarbonate resins have high impact resistance with ductility to notch or crack propagation at an average of up to about 0.2 inches thickness when the incident notch is 10 mils (thousandths) of an inch in radius. Above this average thickness the impact resistance and ductility of polycarbonate resins decline. This phenomenon is commonly found in glassy plastics and is referred to as the critical thickness for notched impact resistance of a glassy plastic.
Additionally, the impact strength of notched polycarbonate resins decreases as temperatures decrease below about -5.degree. C. and also after aging the polymers at elevated temperatures above about 100.degree. C. These temperatures are commonly found in applications where extremes of heat and cold are to be expected.
Thus, it is desirable to use a composition which extends the impact strength and ductility of polycarbonate resins to variable thickness parts or articles of use which resist embrittlement upon exposure to high or low temperatures in a notched or scratched condition.
Compositions are known which extend the high impact resistance and ductility characteristics of polycarbonate resins to parts beyond the critical thickness and under low and high temperature aging conditions, but many of these compositions suffer from incompatibilities of the polymeric components which results in poor weldline and knit line strengths of fabricated parts as evidenced by low double-gate impact strengths when measured according to ASTM D256.
It has been reported in a Research disclosure No. 20810, Dow Chemical Company, August, 1981 that polycarbonates modified with ethylene/octene-1 copolymer provide good impact strength at increased part thickness.
It has now been found that use of a modifier comprising ethylene copolymerized with lower carbon content comonomers, namely, C.sub.4 to C.sub.7 alpha-olefin comonomer, and particularly a linear low density polyethylene made with butene-1 as a comonomer in a polycarbonate resin results in a molding composition that may be utilized to make molded articles having improved properties in comparison with compositions of a polycarbonate and a linear low density polyethylene made with octene-1 as a comonomer. In particular, polycarbonate modified with a copolymer of ethylene and butene-1 has superior properties, e.g., impact strength, in comparison with polycarbonate modified at the same ratio with a copolymer of ethylene and octene-1, as will be shown later. In addition, the aromatic polycarbonate molding compositions made with modifiers comprising linear low density polyethylene including C.sub.4 to C.sub.7 alpha-olefin comonomers generally are less sensitive to property loss under high shear conditions of processing than those made with linear low density polyethylenes including octene-1 as a comonomer. Moreover, molded articles of the above two systems will show distinction favorable to C.sub.4 -C.sub.7 alpha-olefin comonomers when placed under thermal aging system.
Compositions according to the present invention will have a weld line strength according to ASTM D256 of greater than about 7.0 ft./lbs. and preferably above about 9.5 ft/lbs.